1. Technical Field
The present invention relates to a spark plug for an internal combustion engine that is used in the engine of an automobile and the like.
2. Background Art
A spark plug is often used as an ignition means in an internal combustion engine, such as an engine of an automobile. In the spark plug, a center electrode and a ground electrode are placed so as to oppose each other in an axial direction of the spark plug, and a spark discharge gap is formed therebetween. The spark plug generates a discharge in the spark discharge gap, and uses the discharge to ignite an air-fuel mixture inside a combustion chamber.
Here, airflow, such a swirl flow or a tumble flow, of the air-fuel mixture is formed inside the combustion chamber. Ignitability can be ensured as a result of the airflow suitably flowing through the spark discharge gap as well.
However, depending on the attachment position of the spark plug to the internal combustion engine, a portion of the ground electrode joined to the tip portion of a housing may be disposed on the up-stream side of the airflow in the spark discharge gap. In this case, the airflow inside the combustion chamber may be blocked by the ground electrode, and the airflow near the spark discharge gap may stagnate. When this stagnation occurs, the ignitability of the spark plug may decrease. In other words, the ignitability of the spark plug may vary depending on the attachment position to the internal combustion engine. The use of lean-burn internal combustion engines has been increasing particularly in recent years. However, combustion stability may decrease in such internal combustion engines, depending on the attachment position of the spark plug.
In addition, it is difficult to control the attachment position of the spark plug to the internal combustion engine, or in other words, the position of the ground electrode in a circumferential direction. A reason for this is that the attachment position changes depending on the state of formation of attachment screws in the housing, the degree of tightening of the spark plug during the attachment operation to the internal combustion engine, and the like.
Therefore, to suppress obstruction of airflow by the ground electrode, a configuration in which hole-boring machining is performed on the ground electrode and a configuration in which the ground electrode is joined to the housing by a plurality of thin, plate-shaped members are disclosed in PTL 1.